Lineshape Simulations Research Articles

13C-Formate as an indirect low-temperature 1H lineshape polarimeter

1H polarization quantification is important for dissolution-dynamic nuclear polarization (dDNP) but can be cumbersome due to the requirement of acquiring thermal equilibrium signals and measurements that are complicated by large background signals. 1H nuclear magnetic resonance (NMR) spectra can also be deleteriously influenced by line distortions linked with radiation damping from 1H DNP and cannot be used for accurate calculation of 1H polarization. Determining 1H polarization via immediate 13C lineshape analysis of a simple molecule removes such complications. We present 13C-sodium formate as a straightforward system for indirect 1H polarimetry. The 13C NMR spectra acquired under dDNP conditions have distinct features that are readily reproduced with 13C lineshape simulations. 1H polarizations built-up during 1H DNP were indirectly inferred by fitting simulations to 13C lineshapes. We provide the MATLAB scripts used for 13C lineshape analysis in order that the method can be readily implemented in other laboratories.

Rotational Behavior about the N3-Pyridyl Bond in 3-(Pyridin-2-yl)quinazolin-4-one and 4-Thione Derivatives.

The rotational barriers about the N3-(2-pyridyl) bond in 2-iso-propyl-3-(pyridin-2-yl)quinazolin-4-one and the thione analogue were evaluated though VT-NMR measurement of a diastereotopic iso-propyl group followed by a line-shape simulation. In 3-(pyridin-2-yl)quinazoline-4-thione bearing a chiral center as the C2 substituent, the formation of dynamic diastereomers was detected by NMR. The rotational pathway about the N3-(2-pyridyl) bond and the stereochemistries of dynamic diastereomers were revealed through a computational study.

Electron paramagnetic resonance as a tool to determine the sodium charge storage mechanism of hard carbon

Hard carbon is a promising negative electrode material for rechargeable sodium-ion batteries due to the ready availability of their precursors and high reversible charge storage. The reaction mechanisms that drive the sodiation properties in hard carbons and subsequent electrochemical performance are strictly linked to the characteristic slope and plateau regions observed in the voltage profile of these materials. This work shows that electron paramagnetic resonance (EPR) spectroscopy is a powerful and fast diagnostic tool to predict the extent of the charge stored in the slope and plateau regions during galvanostatic tests in hard carbon materials. EPR lineshape simulation and temperature-dependent measurements help to separate the nature of the spins in mechanochemically modified hard carbon materials synthesised at different temperatures. This proves relationships between structure modification and electrochemical signatures in the galvanostatic curves to obtain information on their sodium storage mechanism. Furthermore, through ex situ EPR studies we study the evolution of these EPR signals at different states of charge to further elucidate the storage mechanisms in these carbons. Finally, we discuss the interrelationship between EPR spectroscopy data of the hard carbon samples studied and their corresponding charging storage mechanism.

Ab initio quantum scattering calculations and a new potential energy surface for the HCl(X1Σ+)-O2(X3Σg-) system: Collision-induced line shape parameters for O2-perturbed R(0) 0-0 line in H35Cl.

The remote sensing of abundance and properties of HCl-the main atmospheric reservoir of Cl atoms that directly participate in ozone depletion-is important for monitoring the partitioning of chlorine between "ozone-depleting" and "reservoir" species. Such remote studies require knowledge of the shapes of molecular resonances of HCl, which are perturbed by collisions with the molecules of the surrounding air. In this work, we report the first fully quantum calculations of collisional perturbations of the shape of a pure rotational line in H35Cl perturbed by an air-relevant molecule [as the first model system we choose the R(0) line in HCl perturbed by O2]. The calculations are performed on our new highly accurate HCl(X1Σ+)-O2(X3Σg-) potential energy surface. In addition to pressure broadening and shift, we also determine their speed dependencies and the complex Dicke parameter. This gives important input to the community discussion on the physical meaning of the complex Dicke parameter and its relevance for atmospheric spectra (previously, the complex Dicke parameter for such systems was mainly determined from phenomenological fits to experimental spectra and the physical meaning of its value in that context is questionable). We also calculate the temperature dependence of the line shape parameters and obtain agreement with the available experimental data. We estimate the total combined uncertainties of our calculations at 2% relative root-mean-square error in the simulated line shape at 296K. This result constitutes an important step toward computational population of spectroscopic databases with accurate abinitio line shape parameters for molecular systems of terrestrial atmospheric importance.

Gaussian and Faddeeva broadening of the Franz-Keldysh oscillations at three-dimensional critical-points

Faddeeva broadening (FB) couples the physical characteristics of Gaussian and Lorentzian broadening into a single profile shaping the spectral response of dielectric functions. The electric field induces Franz-Keldysh oscillations (FKOs) as deviations in the dielectric function of semiconductors. We investigated the impact of Gaussian and FB on FKOs at the isotropic M0 of three-dimensional critical points. The twenty-pole Martin–Donoso–Zamudio quadrature was applied to compute the FB of FKOs in differential reflectivity based on the electro-optic function, H(z), with the complex Airy function, Ai(z). H(z) was simplified into a compact product of to reduce the truncation error and determine its asymptotics. We derived a general asymptotic expansion of order N, accounting for Stokes’ phenomenon as the arg(z) crosses The computation of H(z) is detailed for twelve digits of accuracy. The effects of higher Gaussian and FB widths on the damping of FKOs are assessed using FKO peak-to-peak attenuations. For the low-field regime, this work H(z) smoothly evolves into Aspnes’ third derivative functional form (TDFF). Between low- to intermediate-field regimes, we investigated the boundaries for the relevant use of the TDFF by assessing parameters extracted from simulated lineshapes, and the average root mean-squared error. We fitted of the Faddeeva broadened single Airy model to published experimental photoreflectance (PR) lineshapes of InGaAsP. To model degenerate light-and heavy-holes transitions, we fitted of the Faddeeva broadened double Airy model to published experimental contactless electroreflectance (CER) lineshapes of GaAs. In addition to legacy mean-squared error () and coefficient of determination (), the goodness of fit was evaluated using the unbiased Akaike information criteria. The effectiveness of Faddeeva versus Lorentzian broadening in sample PR and CER spectra analysis was assessed using Airy function FKO models. Concludingly, the FB of the differential reflectivity model outperformed the Lorentzian broadening even with the empirical energy-dependent width.

Line-shape analysis and simulation of Er3+ photoluminescence spectra in erbium-stabilized nanocrystalline zirconia structures

Peculiarities of the near-infrared and visible up-conversion spectra of nanocrystalline (Er2O3)x (ZrO2)1-x have been studied. It is shown that for the samples with 0.026 < x < 0.143 the photoluminescence spectra of Er3+ are defined a by similar erbium environment in spite of the different symmetry of the host ZrO2 matrix. The low temperature and room temperature emission bands in the spectral range of the 4I13/2 → 4I15/2 transition are measured and interpreted in terms of the multi-electron molecular orbitals cluster model developed for the case of nanocrystal structures. The role of the intra-manifold 4I13/2 relaxation caused by the spatial confinement effects in erbium-stabilized nanocrystalline zirconia is discussed.

Microwave spectroscopy of positronium atoms in free space

We report measurements of the positronium (Ps) $2^{3}S_{1}\ensuremath{\rightarrow}2^{3}P_{2}$ interval in which atoms traveling in free space were irradiated with microwave radiation generated using a horn antenna. Previous measurements of this transition, performed using atoms in waveguides, exhibited asymmetric and shifted line shapes. In the free-space measurements we report here, much smaller line-shape asymmetry was observed, but with large frequency shifts that varied with the orientation of the horn antenna. Our observations are supported by line-shape simulations and demonstrate that variations in the microwave radiation field distribution can perturb measured line shapes and give rise to apparent frequency shifts without necessarily causing large asymmetries; this effect can explain previous measurements in which an apparent discrepancy with predictions from quantum electrodynamics was ob- served [L. Gurung, T. J. Babij, S. D. Hogan, and D. B. Cassidy, Phys. Rev. Lett. 125, 073002 (2020)].

Laser-induced fluorescence spectroscopy for kinetic temperature measurement of xenon neutrals and ions in the discharge chamber of a radiofrequency ion source

Application of single-photon absorption laser-induced fluorescence (LIF) spectroscopy for non-intrusive measurement of neutral xenon and singly charged xenon ion kinetic temperatures in the discharge chamber of a gridded radiofrequency ion source is demonstrated. A LIF spectrum analysis approach including hyperfine structure reconstruction and inverse filtering (Fourier deconvolution) is outlined. Special focus is set on optimization of post-deconvolution filtering as well as retracing of deconvolution result imperfection due to hyperfine structure parameter uncertainty, incorrect natural linewidth, and saturation of the LIF signal. The corresponding contributions to the kinetic temperature estimation error are quantified via simulation of spectral lineshapes. Deconvolution of almost unsaturated LIF spectra recorded in the center of the ion source discharge chamber reveals that the neutral xenon and xenon ion kinetic temperatures range between approximately 500 and 700 K and, respectively, 700 and 1000 K depending on the radiofrequency power supplied to the discharge.

Nonthermal effects on hydrogen Stark profiles

Plasma waves in nonthermal conditions can be strongly amplified by instabilities. We consider plasma conditions where a velocity-space instability amplifies the oscillating electric field magnitude of electron plasma waves up to several times the average plasma microfield. For a wide range of plasma conditions, the first Lyman and Balmer lines of hydrogen are then affected by both the dynamic plasma microfield and the oscillating field. We present computer simulations of line shapes showing the changes expected in such nonthermal conditions.

Electroreflectance spectroscopy of few-layer MoS2 : Issues related to A1s exciton subspecies, exciton binding energy, and inter-layer exciton

Optical spectra of few-layer transition metal dichalcogenide semiconductors reveal several transitions whose character and origins continue to be debated. We have studied hBN encapsulated few-layer MoS2 films using electroreflectance (ER) spectroscopy. Two strong features are seen in the reflectance spectrum of trilayer MoS2 around the ground state A–exciton transition. In ER, the corresponding features show opposite phase response to the applied voltage. Evidence from first principles ER line shape simulation and photoluminescence spectroscopy suggests that these two features are likely to be A1s exciton subspecies proposed earlier, whose energy depends on which layer the electron–hole pair is located in. The first excited state A2s exciton transition is also identifiable in ER. Through the two-dimensional hydrogenic exciton model, it enables an approximate estimation of the exciton binding energy Eb. The increase in Eb with decreasing film thickness, which originates from reduced dielectric screening, is phenomenologically analyzed through a film thickness and capping material dependent effective dielectric constant. Extending this idea, we show that the A1s exciton is mostly confined to a single S–Mo–S layer, as predicted by theory. An inter-layer (IL) exciton is expected in bilayer and thicker 2H-MoS2 films. However, we show that there can be bilayer films where the IL exciton is absent, which may be related to increased carrier concentration.

Intermolecular Charge Transfer in H- and J-Aggregates of Donor–Acceptor–Donor Chromophores: The Curious Case of Bithiophene-DPP

A vibronic exciton model is developed to describe low-energy electronic excitations in slip-stack aggregates of donor–acceptor–donor (DAD) chromophores with substantial overlap between the neighboring donor and acceptor fragments. In such stacks, J- and H-aggregate behavior is driven by intermolecular charge transfer (ICT) and not Coulomb coupling as is assumed in the Kasha model. In-phase (out-of-phase) intermolecular charge transfer integrals result in J-aggregate (H-aggregate) behavior, as unambiguously determined by the vibronic spectral signatures. Interestingly, both J- and H-aggregates are red-shifted, in contrast to the predictions of the Kasha model. Simulated spectral line shapes agree well with recent experiments on two derivatives of the bithiophene diketopyrrolopyrrole chromophore, 2T-DPP-2T, with different terminal groups.

Quantum chemical elucidation of a sevenfold symmetric bacterial antenna complex

The light-harvesting complex 2 (LH2) of purple bacteria is one of the most studied photosynthetic antenna complexes. Its symmetric structure and ring-like bacteriochlorophyll arrangement make it an ideal system for theoreticians and spectroscopists. LH2 complexes from most bacterial species are thought to have eightfold or ninefold symmetry, but recently a sevenfold symmetric LH2 structure from the bacterium Mch. purpuratum was solved by Cryo-Electron microscopy. This LH2 also possesses unique near-infrared absorption and circular dichroism (CD) spectral properties. Here we use an atomistic strategy to elucidate the spectral properties of Mch. purpuratum LH2 and understand the differences with the most commonly studied LH2 from Rbl. acidophilus. Our strategy exploits a combination of molecular dynamics simulations, multiscale polarizable quantum mechanics/molecular mechanics calculations, and lineshape simulations. Our calculations reveal that the spectral properties of LH2 complexes are tuned by site energies and exciton couplings, which in turn depend on the structural fluctuations of the bacteriochlorophylls. Our strategy proves effective in reproducing the absorption and CD spectra of the two LH2 complexes, and in uncovering the origin of their differences. This work proves that it is possible to obtain insight into the spectral tuning strategies of purple bacteria by quantitatively simulating the spectral properties of their antenna complexes.

Serotonergic drugs modulate the phase behavior of complex lipid bilayers

Serotonin is an endogenous neurotransmitter involved in both physiological and pathophysiological processes. Traditionally, serotonin acts as a ligand for G protein-coupled receptors (GPCRs) leading to subsequent cell signaling. However, serotonin can also bind to lipid membranes with high affinity and modulate the phase behavior in 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC)/N-palmitoyl-D-erythro-sphingosylphosphorylcholine (PSM)/cholesterol model membranes mimicking the outer leaflet of the plasma membrane. Here, we investigated if serotonergic drugs containing the pharmacophore from serotonin would also modulate phase behavior in lipid membranes in a similar fashion. We used 2H NMR spectroscopy to explore the phase behavior of POPC/PSM/cholesterol (4/4/2 molar ratio) mixtures in the presence of the serotonergic drugs aripiprazole, BRL-54443, BW-723C86, and CP-135807 at a lipid to drug molar ratio of 10:1. POPC and PSM were perdeuterated in the palmitoyl chain, respectively, and prepared in individual samples. Numerical lineshape simulations of the 2H NMR spectra were used to calculate the order parameter profiles and projected lengths of the saturated acyl chains. All serotonergic drugs induce two components in 2H NMR spectra, indicating that they increased the hydrophobic mismatch between the thickness of the coexisting lipid phases leading to larger domain sizes, relatively similarly to serotonin. AFM force indentation and Raman spectral studies, which interrogate membrane mechanical properties, also indicate changes in membrane order in the presence of these drugs. These findings highlight how serotonergic drugs alter membrane phase behavior and could modulate both target and other membrane proteins, possibly explaining the side effects observed for serotonergic and other clinically relevant drugs.

CW-EPR spectra of P1 centers in HPHT diamond in the vicinity of Rabi resonance: possible standard for [formula omitted] evaluation in EPR and DNP enhanced NMR spectroscopies

Synthetic commercial type-IIa diamond crystal containing low concentration of P1 (Ns0) centers ([Ns0] <1ppm) was examined as a possible calibration standard for evaluating microwave magnetic field intensity (B1) in the spectrometer cavity. The reliability of this standard was experimentally and theoretically tested by employing two different types of microwave cavities with different distributions of B1 values. The procedure relies on the ordinary continuous wave electron paramagnetic resonance (CW-EPR) with magnetic field modulation (ωrf/2π=100 kHz) applied under the regime of the microwave power (PMW) saturation of the out-of-phase first harmonic signal. The power saturation procedure for PMW∼B11/2 was analyzed with respect to the change of spectral lineshape when Rabi frequency (ω1=γB1, γis the electron gyromagnetic ratio) approaches to ωrf. The phenomenon of the inversion of sideband lines while passing the Rabi resonance condition (ω1=ωrf) was analyzed. In specific, from the simulation of the experimental lineshapes in the close vicinity of Rabi resonance (ω1≈ωrf) the correspondingB1 can be assigned. For the chosen diamond crystal low value of B1≈3.6µT at Rabi resonance was determined what makes it a convenient standard for DNP enhanced NMR spectroscopy which requires relatively small B1.

Investigation of CO2 Orientational Dynamics through Simulated NMR Line Shapes*.

The dynamics of carbon dioxide in third generation (i. e., flexible) Metal‐Organic Frameworks (MOFs) can be experimentally observed by 13C NMR spectroscopy. The obtained line shapes directly correlate with the motion of the adsorbed CO2, which in turn are readily available from classical molecular dynamics (MD) simulations. In this article, we present our publicly available implementation of an algorithm to calculate NMR line shapes from MD trajectories in a matter of minutes on any current personal computer. We apply the methodology to study an effect observed experimentally when adsorbing CO2 in different samples of the pillared layer MOF Ni2(ndc)2(dabco) (ndc=2,6‐naphthalene‐dicarboxylate, dabco=1,4‐diazabicyclo‐[2.2.2]‐octane), also known as DUT‐8(Ni). In 13C NMR experiments of adsorbed CO2 in this MOF, small (rigid) crystals result in narrower NMR line shapes than larger (flexible) crystals. The reasons for the higher mobility of CO2 inside the smaller crystals is unknown. Our ligand field molecular mechanics simulations provide atomistic insight into the effects visible in NMR experiments with limited computational effort.

All-experimental analysis of doubly resonant sum-frequency generation spectra: Application to aggregated rhodamine films.

A new method is proposed to analyze Doubly Resonant infrared-visible Sum-Frequency Generation (DR-SFG) spectra. Based on the transform technique, this approach is free from assumptions about vibronic modes, energies, or line widths and accurately captures through the overlap spectral function all required aspects of the vibronic structure from simple experimental linear absorption spectra. Details and implementation of the method are provided along with three examples treating rhodamine thin films about one monolayer thick. The technique leads to a perfect agreement between experiment and simulations of the visible DR-SFG line shapes, even in the case of complex intermolecular interactions resulting from J-aggregated chromophores in heterogeneous films. For films with mixed H- and J-aggregates, separation of their responses shows that the J-aggregate DR-SFG response is dominant. Our analysis also accounts for the unexplained results published in the early times of DR-SFG experiments.

Pneumatic angle adjustment for magic angle spinning spherical rotors

Precise alignment of the sample's spinning axis is vital for many magic angle spinning solid state NMR experiments. Spherical rotors are a new paradigm for magic angle spinning NMR, having been demonstrated to spin stably with little risk of rotor crash, but like cylindrical rotors, they have previously only utilized a mechanical adjustment method to set the sample's rotation axis to the magic angle. Here we show that by using a second gas aperture within the stator, a spherical rotor's axis of rotation may be precisely pitched about the magic angle without mechanical adjustment or motion of the stator. The 2H MAS sideband lineshape of the carboxylic acid deuteron resonance in d4-malonic acid is used as a measure of the rotor's absolute deviation above or below the magic angle by comparing the experimental data to simulated lineshapes. We observe a linear, monotonic relationship between the angle adjustment gas flow rate and the rotor's pitch angle, which is also accompanied by an increase in spinning rate. Precise and in operando control of the spinning axis angle is expected to have considerable advantages to future implementation of MAS within narrow-bore magnets, and for variable angle spinning (VAS) and dynamic angle spinning (DAS) experiments.

Towards subpicosecond pulses from solid target plasma based seeded soft X-ray laser.

We investigate the coherence of plasma-based soft X-ray laser (XRL) for different conditions that can alter the electron density in the gain region. We first measure the source temporal coherence in amplified spontaneous emission (ASE) mode. We develop a data analysis procedure to extract both its spectral width and pulse duration. These findings are in agreement with the spectral line shape simulations and seeded operation experimental results. Utilizing the deduced spectral width and pulse duration in a one-dimensional Bloch-Maxwell code, we reproduce the experimental temporal coherence properties of the seeded-XRL. Finally, we demonstrate efficient lasing in ASE and seeded mode at an electron density two times higher than the routine conditions. In this regime, using Bloch-Maxwell modeling, we predict the pulse duration of the seeded XRL to be ∼500fs.

193 nm excimer laser-induced color centers in Yb3+/Al3+/P5+-doped silica glasses

Yb3+, Yb3+/Al3+, and Yb3+/P5+ co-doped silica glasses, as well as pure silica, Al3+, and P5+ single-doped silica glasses, were prepared using a sol–gel method combined with high-temperature sintering. The glasses were irradiated with a 193 nm ArF excimer laser for different durations. The species of the 193 nm laser-induced color centers were identified by radiation-induced absorption (RIA), in-situ photoluminescence (PL), and continuous-wave electron paramagnetic resonance (CW-EPR) spectroscopic techniques. To obtain the RIA and CW-EPR characteristics of each color center, the Gaussian decomposition of RIA and the line-shape simulation of the CW-EPR spectra were performed. The formation mechanism of the color centers was correlated with the glass microscopic structure, which was obtained by nuclear magnetic resonance (NMR) spectroscopy. The results show that the oxygen hole center and the Yb2+ ion pairs are primarily responsible for the radiation-induced darkening, and their formation is highly dependent on the charge balance between the Yb3+ ion and its ligand. This work provides insights into the underlying mechanism of pump light and/or external radiation-induced optical losses in Yb3+-doped silica fibers.

Quantifying the statistical noise in computer simulations of Stark broadening

Computer simulations employed in Stark broadening calculations are reexamined within the perspective of giving error bars to results. As a rule, the calculated spectra exhibit a noisy structure, which is inherent to the random number generators involved in the numerical method. Using a variance estimator, we quantify the statistical noise on simulated line shapes. Two expressions for the radiated power spectrum, which are analytically equivalent but lead to different noise levels, are considered. A discussion of this difference is carried out based on an analytical model.